Marked increase in membranolytic selectivity of novel cyclic tachyplesins constrained with an antiparallel two-beta strand cystine knot framework.

We have developed a highly constrained 18-residue cyclic peptide template based on the antimicrobial peptide tachyplesin-1 that features an end-to-end peptide backbone and a cystine knot-like motif with three evenly spaced disulfide bonds to cross-brace the antiparallel beta-strands and to approximate an amphiphatic "beta-tile"-like structure. Six beta-tile analogs were prepared to correlate different topological patterns with membranolytic specificity. Their conformations and antimicrobial and hemolytic activities were compared with tachyplesin-1 and the recently discovered Rhesus monkey theta defensin (RTD) which contains similar beta-tile structural elements. The beta-tile peptides and RTD retained broad spectrum antimicrobial activities. In general, they were less active than tachyplesin-1 in 10 tested organisms but their activity increased under high-salt (100 mM NaCl) rather than in low-salt conditions. The beta-tile peptides are highly nontoxic to human erythrocytes with EC(25) ranging from 600 to 4000 microM. Collectively, our results show that the design of a highly rigid peptide template is useful for further analog study to dissociate antimicrobial activity from cytotoxicity which would be helpful in discovering clinical applications for peptide antibiotics.